Yes but RDRAND is easy to hide. No test can tell the difference between a good and a backdoored RNG, you don't know what to look for, the whole point of randomness is that you don't know what it will do, so you can only trust reasoning based on understanding where the numbers come from.

I'm pretty sure rdrand is fine, and would not bat an eye if Linux trusted it, but I'm not sure I would bet some nuclear launch codes on it. Linux is trying to be as safe as practically possible, not just "Probably safe".